Xerographic development apparatus

ABSTRACT

Apparatus for developing an electrostatic latent image on a moving photoconductive surface in which the surface is moved in contact with the two-component developer material so that an uphill flow of developer material is established therein, the apparatus having means associated therewith to aid the flow of developer material through the system.

United States Patent 11/1959 Stavrakis et 77 775 66 66 7 //9// 3 oooo88., ll 11 ll 11H8 11 taw w trfue SBeE ntbb mmm a l.lr.a HMLLTR 2257666666 999999 HHHHHH 9 2082 1 1 4267483 602 49 7 ,3 1 088 986 902590 N wvJ 2233333 E n .m m" w Pa .m mm C Wm 1 Mn 6 .3 0 99 If 1 hec u HMl b nmh n r 0- 9 PRJK6JA S r. 0 d m a. e mmm W. fla .1 APP 1 1.11 2 25 7 2241 1.11

d n a e t a 6 P a R L S e m n m vl m1 ea M 0 m5 PE m r e .1 H 06 m b a.1 x Z Es won. 0"" m m o n n PA n 0 U. mX o w r F 6 mh em XR e e n .m SS A H 7 .1

[54] XEROGRAPHIC DEVELOPMENT APPARATUS 1 Claim, 4 Drawing Figs.

ABSTRACT: Apparatus for developing an electrostatic latent image on amoving photoconductive surface in which the sur- N b8 51 01 B mh m d Ldha. 1] 0 55 1 face is moved in contact with the two-component developer1 material so that an uphill flow of developer material is having meansassociated established therein, the apparatus therewith to aid the flowof developer material through the system.

PATENTEB we H97! 3,599,604

sum 1 OF 2 INVENTOR. :7 3 PRABHULAL P. CHAWDA BY JOHN P. CALDWELL wwkRAMER Mm ATTORNEYS PATENTEU AUG 1 7 I97! SHEET 2 BF 2 DEVELOPMENT ZONE4 INVEN'I'OR.D

PRABHULAL P CHAW A PRIOR ART JOHN F. CALDWELL BY w| |AM E. KRAMERXEROGRAPHIC DEVELOPMENT APPARATUS This invention relates in general toxerography and, in particular, apparatus for developing a xerographicimage.

In the art of xerography, as originally disclosed by Carlson in U.S.Pat. No. 2,297,691, a plate, comprising a conductive backing upon whichis placed a photoconductive insulating material, is charged uniformlyand the photoconductive surface then exposed to a light image of anoriginal object to be reproduced. The photoconductive coating is causedto become conductive under the influence of the light image so as toselectively dissipate the electrostatic charge found thereon toproducewhat is known as a latent electrostatic image. The development of thelatent image is generally effected by electrostatically attracting apigmented resin to the image areas on the plate. The amount of chargefound in the image areas determines the amount of resin materialattracted thereto. The amount of charge can be said to be proportionalto image density and therefor areas of small charge concentration becomeareas of low toner density while the areas of greater chargeconcentration become proportionally more dense. A permanent record ofthe original object is obtained by transferring the developed image to afinal support material and fixing the developed image thereto.

A wide variety of pigmented resins have been developed for the purposeof developing a latent electrostatic image, these resins being commonlyreferred to as toners." The toner material is generally transported tothe image areas by means of a relatively coarser material known ascarrier, the carrier being adapted to support a quantity of toner uponits surface. The two-component, that is, the toner and carrier, areselected so that the materials interact electrostatically when placed inclose rubbing contact to cause a triboelectric attraction therebetween.This two-component material is known in the xerographic art as developermaterial and this term is used herein to denote a two-componentdeveloper comprising carrier and toner material.

Although many workable xerographic development systems are known in theart, most of these systems have been found to be impracticable, in acommercial sense, because they are either too slow, too inefficient, ortoo complex to readily lend themselves to use in automatic xerographicmachines. Cascade development, as illustrated in U.S. Pats. to Walkup etal., 2,573,881 and to Carlson, 2,990,278, because of its manyadvantages, has become one of the most prevalent methods for developinga latent electrostatic image. In a cascade developing system,two-component developer material is conveyed, as for example by buckets,to a point above an imagebearing xerographic plate and the developermaterial poured or cascaded over the plate surface. Through the combinedmechanical and electrostatic forces involved, toner is dislodged fromthe carrier material and attracted to the image areas on thephotoconductive plate. The cascade system, however, has proven to bespace consuming. The need for bulky conveyors or the like add greatly tothe size of the xerographic-reproducing apparatus. Unwanted powderclouds also result due to the relatively violent cascade action which,in turn, deposits unwanted toner material in background areas.Furthermore, dropping or cascading the developer material on the platesurface causes plate abrasions resulting in a relatively high rate ofplate failures. A high rate of developer material failure due to beadfracture is also noted in the cascade system.

In order to overcome some of the disadvantages found in a prior art and,particularly, those associated with cascade development, a new method ofxerographic development was devised in which a moving photoconductiveplate surface is brought into contact with a quantity of developermaterial contained in a housing or the like. Fundamentally, an uphillflow of developer material is established at the platedeveloperinterface due to the frictional forces involved. These frictional forcesare sufficient to carry the developer material along in contact with themoving surface at approximately drum speed. Although not clearlyunderstood, it is believed that development is effected during theperiod of flowing contact by means of the classicaldevelopment-scavenging technique as disclosed in the previouslymentioned Walkup and Carlson patents. The developer material, upon beingreleased from the plate surface, returns to the backside of thedeveloper housing where it is replenished before once again beingreturned to the zone of active development. This flowcontact-type systemis disclosed in a copending application to Gundlach, Ser. No. 528,846.

Many geometric configurations are discussed and disclosed in theGundlach application, however, they all employ the same basicdevelopment principle of development. Although the basic flow-contactsystem disclosed by Gundlach overcomes some of the previously mentioneddisadvantages found in the prior art, the flow-contact system hascertain inherent disadvantages. The flow-contact system is basically aslow system in that a relatively small volume of material is movedthrough the system during each developing cycle. Addition of new tonermaterial to basic flow-contact housing has also proven to be a majorproblem primarily due to the extremely low flow rates maintainedtherein. It has been found that a preponderance of new toner added tothe housing stagnates in voids and pockets and never finds its way intothe flow stream. This stagnation is believed to be due to the lack ofagitation found in the slow, gentle developer flow.

It is therefore a primary object of this invention to improvexerographic development apparatus.

Yet another object of this invention is to improve the efficiency ofaxerographic development apparatus.

Another object of this invention is to eliminate space charge build upin a flow-contact development system.

It is yet another object of this invention to improve flowcontactdevelopment.

It is yet another object of this invention to improve toner distributionand mixing throughout a flow-contact development system.

It is still another object of this invention to increase the developerflow rate in a flow'contact system.

It is another object of this invention to reduce the amount of timerequired to bring a flow-contact development system to optimum operatingconditions.

These and other objects of the present invention are at tained by meansof a developer housing being capable of holding a quantity oftwo-component developer material having an opening for receiving amoving photoreceptive surface therein, the surface being moved incontact with developer material to establish a flow pattern within thedeveloper housing and a vane-type mixer positioned in the housing sumparea to aid the flow of developer materials in said housing and to mixthe developer material.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is had to the following descriptionof the invention to be read in conjunction with the drawings, wherein:

FIG. 1 is a isometric view ofa developer system suitable for use with axerographic drum, the system embodying the present invention and havingportions thereof broken away to show the internal construction of theapparatus;

FIG. 2 illustrates schematically a preferred embodiment of thedevelopment apparatus of the instant invention adapted for continuoususe in an automatic xerographic machine;

FIG. 3 is a sectional view of the development apparatus taken along line3 3 of FIG. 2 with the xerographic drum shown invisible;

FIG. 4 is an illustration of the basic flow-contact configuration foundin the prior art.

A brief discussion of basic flow-contact development and the flowmechanism associated therewith is believed warranted at this time inorder to more fully understand the teachings of the present invention.In the basic system as shown in FIG. 4, drum 10 is rotatably mounted soas to move in the direction indicated through a clam shell shapedhousing 13. The housing contains sufficient developer material so thatthe drum contacts the developer material as it moves through the housingto establish a flow pattern therein. This flow of material is depictedby the arrows associated therewith. A thin layer of developer materialadjacent to the drum surface moves in an uphill direction atapproximately drum speed while the entire back layer of developer (thebackside of the system) moves down more or less as a unit to fill thevoid left by the upwardly moving material.

This uphill flow of developer material in contact with the drum surfacedefines what is herein referred to as the active development zone.Theoretically, properly charged and toner leaded developer material isdelivered to the start of the active development zone from the supply ofdeveloper material found on the backside of the system. The developermaterial introduced into the active zone is carried along in contactwith the drum surface as the drum rotates where development takes placeby means of the classical xerographic development-scavenging mechanism.The developer material continues to flow along in contact with the drumuntil the frictional forces holding the developer at the interface areovercome, as for example the drum leaving the sump area. The developermaterial leaving the active developing zone returns to the backside ofthe system to fill the void created by the upward flow of material. Ascan be seen, the carrier beads leaving the active development zone havegiven up their toner in the development process and therefore must bereplenished on the backside of the system before they can once again bereturned to the start of the active development zone.

It has been found that the flow rate in the basic flow-contact-typesystem is not limited by the drum developer interface motion but ratherby the time required to return replenished developer material to theactive development through the backside of the system. Because this is aclosed system, the volume rate of flow through the active developmentzone is equalto the rate of flow found on the backside of the system. Infact tests have shown that by providing an infinite supply of developermaterial to the start of the active development zone, a flow rateapproximately eight times greater than found in the basic C-shell systemcan be attained.

From the prior discussion, it should be clear that the backside of thedeveloper system should have the steepest return geometry possible inorder to provide the more rapid flow rate through the system. However,the angle of repose of the developer material acts to limit the returngeometry of the system. The angle of repose is the maximum angle withthe horizontal at which a quantity of loose particulate material willretain its position before sliding. As shown in FIG. 4, the angle ofrepose of the developer material within the developer housing 12precludes the developer from contacting the drum surface for aconsiderable distance as the drum rotates through the housing. In fact,for most commercially available particulate developer materials, contactbetween drum and developer is made at about 45 from the vertical. As canbe seen, the material angle of repose severely limits the length of theactive development zone and severely restricts the return geometry ofthe system.

It should be noted, however, that basic flow-contact development isnevertheless a highly efficient system. With a volumetric flow ratesomewhere in the nature of between 200 and 400 times less than the flowrate found in most commercially available development systems, as forexample cascade, the basic flow-contact system is able to produce goodcopy by efficiently utilizing the developer available. This highefficiency is evidenced by the extremely starved condition of thecarrier beads as they leave the active development zone.

The apparatus of the present invention for improving xerographicdevelopment system as well as eliminating the difficulties associatedtherewith is shown in FIGS. 1, 2, and 3. Although it will becomeapparent that the instant invention is well adapted for use in anysuitable xerographic reproduction apparatus, it is shown herein embodiedin a drum-type xerographic apparatus for purposes of illustration. Asshown in FIG. 2, drum 10 is mounted on shaft Ill and the shaft rotatablysupported in the frame of the xerographic machine (not shown). The majorxerographic processing components are mounted around the drum peripheryso that they are able to act thereon as the drum continually rotatesthrough the various stations.

In general, the several xerographic processing stations in the path ofmovement of the drum surface may be described functionally as follows:

a charging station A, at which a uniform electrostatic charge isdeposited on the surface of the photoconductive drum;

an exposure station B, at which a light or radiation pattern of a copyto be reproduced is projected onto the xerographic drum to dissipate thecharge in the exposed areas to form a latent electrostatic imagethereon;

a developing station C, at which the xerographic developing material,including toner particles having an electrostatic charge opposite tothat of the electrostatic image, are placed in contact with the movingdrum surface whereby the toner particles are caused to adhere to theelectrostatic latent image found thereon;

a transfer station D, where the developed electrostatic image istransferred from the plate surface to a final support material, and;

a drum cleaning and charging station E, at which the plate surface isbrushed to remove residual toner particles remaining thereon after imagetransfer and at which time the plate surface is exposed to a relativelyintense light source to effect substantially complete discharge of anyresidual electrostatic charge found thereon.

In the apparatus of the instant invention, housing 20 is formed toprovide an extremely steep return geometry through which flows starveddeveloper material 15 leaving the active development zone. Rotatablymounted in the lower portion of the developer housing is a vane mixer21. It should be noted that the impeller 31 of the vane mixer act notonly like a pump impeller to move the material along but also acts as amixing device to rapidly and effectively mix and triboelectrify materialacted upon.

The top portion of housing 20 has an opening for receiving in rotatablerelationship therein a portion of drum surface 10. The housing, and thetop opening therein, extend at least the longitudinal length of the drumand the housing is closed at each end by means of end plates 25 and 26,respectively, (FIG. 3). Seals 28 are mounted between the drum and theend plates to prevent developer material from falling therebetween. Inoperation, sufficient developer material is placed within housing 20 sothat the rotating drum surface, as it passes through the housing, isbrought into moving contact with the developer. The region adjacent tothe rotating drum, in which the developer material contacts thephotoconductive surface, describes the active development zone in theinstant invention.

Vane mixer 21 comprises a shaft portion 30 and an impeller portion 31,the impeller portion having paddle wheellike impeller blades mountedthereon which extend at least as long as the longitudinal length of thedrum surface. The vane mixer is rotatably mounted in the lower portionof the housing 20 in bearing blocks 32 provided in end plates 25 and 26,respectively, and is driven through means of a sprocket 35 rigidlyaffixed to the extended portion of shaft 30. As shown in FIG. 2, theimpeller is driven in a clockwise direction by any suitable drive means,as for example motor 36, acting through sprocket 35.

In operation, toner-loaded carrier material enters the start of theactive development zone at approximately'point M and the developermaterial is carried along in contact with the photoconductive surface asthe drum rotates in the direction indicated. The developer materialbrought in contact with the drum developes the electrostatic latentimage thereon through the classical development-scavenging mechanism asdescribed in the previously mentioned Walkup patent. That is, the electrostatic latent image possessing a higher electrostatic attraction thanthat possessed by the carrier material causes the toner to be attractedfrom the carrier onto the drum surface in the image areas. The developermaterial continues to move in friction contact with the drum surfaceuntil the friction force is by the drum moving out of contact with thedeveloper in the sump of the housing. This point is marked N FIG. 2. Thetoner-starved carrier falls into the void created on the backside of thesystem by the continuous stream of new material entering the activedevelopment zone.

Back plate 36 of housing is mounted at a substantially small angle withthe vertical so that a relatively steep return geometry is seen by thedeveloper material leaving the active development zone at point N.However, as previously noted, the flow of developer material through aC-shell system is not basically dependent upon the geometry of thesystem. A relatively shallow flow pattern, as described in FIG. 4, wouldbe established within housing 20 if it were not for the pressure headadded to the system by means of vane mixer 21. Impeller 31 rotating inthe direction indicated, pumps the developer material from the bottom ofhousing 20 towards the start of the active development zone at point M.The pump action of the vane mixer displaces developer material long thebottom portion of back plate 36 creating a void which is quickly filledbecause of the steep geometry of the system. Starved toner beads leavingthe active development zone now are forced to follow the steep geometryof the housing rather than the naturally shallow return found in thebasic C-shell pattern thus establishing a more rapid flow within thesystem.

The active development zone in a flow-contact-type system should be ofsufficient length in order for the carrier beads to give up toner in thedevelopment process. In a flow system, as herein described, the lengthof the active development zone determines the amount of time thedeveloper is in contact with the image. As previously noted, flowcontact development is an extremely efficient system in which a largepercentage of toner material on the carrier is given up to the latentelectrostatic image thereby leaving the carrier substantially denuded. Adenuded or starved carrier head in the active development zone tends toact as a scavenger in that it ceases to act as a donor and acts to pulltoner away from the developed xerographic images. It is thereforedesirable in a flow-contact system to control the length of the activedevelopment zone to minimize the scavenging effect.

A roll 42 positioned in toner hopper 40 is rotatably mounted in bearingblocks 43 and the bearing blocks positioned in end plates and 26,respectively. In this preferred embodiment, roll 42 is a smooth surfacestainless steel roll which is rotated through the toner hopper at aspeed to produce sufficient mechanical action to carry the finelydivided toner material into the main housing 20. Roll 42 is positionedin opening 41 so that an aperture is formed between the roll on thesidewall of the opening such that toner can pass therebetween but thecoarser carrier material is precluded from backing up into the tonerhousing. Although not necessary, a fine mesh screen 44 is alsopositioned adjacent to the said aperture. on the developer housing sideof opening 41 through which toner material to pass into the main flowstream of the developer material but prevents the carrier material fromflowing in contact with the rotating roller 42.

As shown in FIG. 3, the shaft portion of roller 42 extends exterior tothe developer housing and has rigidly affixed thereto gear 38. Gear 38,in turn, is matted to the drive gear on the vane mixer. The gear ratiobetween the matting, gears is preselected so that roller 42 rotates at aspeed so that toner addition is compatible with the flow rate beingmaintained by the vane mixer 21. Increasing the speed ofthe vane mixerwill, in effect, cause more toner to be added to the system thus makingthe system self-regulating.

Space charge build up around the carrier material is entirely eliminatedin the present invention by placing the vane mixer at a groundpotential. In order for developer material to reach the activedevelopment zone, the developer material must be acted upon by the vanemixer 21. It has been found that the grounded vane mixer efficientlybleeds off any space charge build up that might occur in the backside ofthe system thereby assuring that only properly charged developermaterial reaches the start ofthe active development zone.

The apparatus of the present invention lends itself to use in automaticxerographic apparatus not only because of its high efficiency but alsobecause of its rapid response upon start up. The vane mixer system, asherein disclosed, when properly loaded with developer material reachesoptimum operating conditions in a matter of seconds. This ability torapidly reach operating conditions gives the present apparatus thecapabili ties needed in rapid automatic xerographic machines.

While the invention has been described with reference to the structuredisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications and/or changes asmay come within the purposes of the improvements of the scope of thefollowing claims.

What we claim is:

1. In a copying apparatus in which a latent electrostatic image on aphotoconductive surface in the form ofa rotatably mounted xerographicdrum is rotated at a predetermined speed in an upheld direction past adevelopment station positioned at the lower quadrant thereof, thedevelopment station including a housing having an opening to receive aportion of the rotating drum therein, wherein the housing is adapted tohold a quantity of two component developer material including carrierand toner particles in contact with the portion of the drum surfacealong an active development zone whereby a flow of developer material ismoved in contact with the drum surface as the drum rotates through thehousing, an improved development apparatus comprising impeller meanspositioned in said housing for pumping developer material from thebottom portion of the housing toward the start of the active developmentzone, means to drive said impeller means relative to the rotating drumsurface at a speed sufficient to maintain a proper flow of developermaterial through said housing in contact with the drum surface, means tomaintain said impeller at a ground potential during development toensure that properly charged developer material is introduced at thestart of the active development zone continuously, and hopper meanslocated on the downhill side of the developer flow in directcommunication with the developer flow within the housing for introducingadditional toner particles to the developer material as toner particlesare consumed during development.

1. In a copying apparatus in which a latent electrostatic image on aphotoconductive surface in the form of a rotatably mounted xerographicdrum is rotated at a predetermined speed in an upheld direction past adevelopment station positioned at the lower quadrant thereof, thedevelopment station including a housing having an opening to receive aportion of the rotating drum therein, wherein the housing is adapted tohold a quantity of two component developer material including carrierand toner particles in contact with the portion of the drum surfacealong an active development zone whereby a flow of developer material ismoved in contact with the drum surface as the drum rotates through thehousing, an iMproved development apparatus comprising impeller meanspositioned in said housing for pumping developer material from thebottom portion of the housing toward the start of the active developmentzone, means to drive said impeller means relative to the rotating drumsurface at a speed sufficient to maintain a proper flow of developermaterial through said housing in contact with the drum surface, means tomaintain said impeller at a ground potential during development toensure that properly charged developer material is introduced at thestart of the active development zone continuously, and hopper meanslocated on the downhill side of the developer flow in directcommunication with the developer flow within the housing for introducingadditional toner particles to the developer material as toner particlesare consumed during development.